1. Technical Field
The present disclosure relates to sound synthesis.
2. Introduction
Frequency Modulation Synthesis and Phase Modulation Synthesis—both commonly referred to as “FM Synthesis”—are well known and widely used. John Chowning first described the technique in his article “The Synthesis of Complex Audio Spectra by the Means of Frequency Modulation” in 1973. In one form, this type of modulation synthesis consists of two sine oscillators—one is called a “Carrier” and one a “Modulator”. An output signal of the modulator changes the phase or frequency of the carrier, and the carrier's output signal is used as an audio signal.
The spectral content of such an FM configuration is controlled by the carrier frequency, the modulator frequency and the modulation intensity or amplitude. If carrier and modulator frequency are both integer multiplies of a common fundamental frequency, then the spectrum will be purely harmonic. Otherwise the spectrum will also have inharmonic content. Inharmonicity refers to an amount of inharmonic content in a spectrum.
The ratio between carrier and modulator frequency has a strong impact on the frequency range that is most prominent. Continuously changing the modulator frequency by sweeping a formant through frequencies leads to wide areas of inharmonic sounds and only small regions of mostly harmonic results because, under current technology, the modulator/carrier frequency ratio and amount of inharmonic content in a spectrum are linked.